In recent years, the environmental problem caused by volatile organic compounds contained in, for example, the air has been worsened. As specific examples thereof, there can be cited a case where it is pointed out that formaldehyde generated from adhesives or preservatives for building materials induces a sick building syndrome in indoor spaces, a case where acetaldehyde generated from, for example, the remainder in food factories causes a bad smell in industrial worlds, and so forth.
Conventionally, the technique has been carried out wherein the air is purified using an air filter including an adsorbent such as activated carbon to thereby adsorb volatile organic compounds. However, in case of the activated carbon, a problem has been pointed out that as the adsorbed organic compounds cover active sites of the activated carbon, its adsorbing activity is gradually lowered. In view of this, there has been proposed a technique of purifying the air by decomposing volatile organic compounds using, as one of purifying means in place of activated carbon, a photocatalyst that is activated when applied with light to exhibit oxidation-reduction activity, i.e. so-called photocatalytic activity.
As one example of the foregoing photocatalyst, there has been known such a photocatalyst composed of titania being titanium oxide having a property of exhibiting photocatalytic activity, and a carrier such as silica carrying titania thereon (hereinafter, this photocatalyst will be referred to as “titania/silica catalyst”). The titania/silica catalyst has a strong oxidation power, while its reduction power is slightly inferior to the oxidation power. It has been known that when platinum (Pt) is further supported on the titania/silica catalyst, there can be obtained a photocatalyst (hereinafter referred to as “platinum-titania/silica catalyst”) whose oxidation-reduction activity is further enhanced. (e.g. see Non-patent Literature 1).
When purifying the air containing volatile organic compounds such as acetaldehyde by the use of the foregoing platinum-titania/silica catalyst, light from a light source such as a blacklight is applied to a reactor filled with a layer of photocatalyst to thereby activate the photocatalyst, and simultaneously, the air is fed to the photocatalyst by supply and exhaust means such as a fan. In this event, acetaldehyde is decomposed into, for example, carbon dioxide and water by the photocatalytic activity so that the air is purified.
[Non-patent Literature 1]
E. Obuchi, T. Sakamoto, and K. Nakano, “Photocatalytic Decomposition of Acetaldehyde over TiO2/SiO2 Catalyst”, Chemical Engineering Science 54 (1999), pp. 1525-1530, Mar. 26, 1999
However, there arises a problem that the decomposition of acetaldehyde by the foregoing photocatalyst is insufficient. Specifically, although most of acetaldehyde can be decomposed by the photocatalyst, acetaldehyde may make a human being feel uncomfortable due to its strong offensive smell even if it remains at several tens of ppm. Further, when the catalyst activity is low, there are those instances where acetaldehyde is not decomposed into carbon dioxide and water so that another odor substance such as formic acid or acetic acid being an intermediate substance is produced. Therefore, the fact is that when purifying the air in, for example, an indoor space, the indoor air is circulated to the photocatalyst to lower the concentration of acetaldehyde or the intermediate substance over a certain long time. To this end, there have been demanded techniques that can quickly purify the air at a higher decomposition rate and, as one of them, higher activation of the photocatalyst has been considered.
As another problem, if coke being the so-called remainder produced upon decomposition of acetaldehyde adheres to the surfaces of the catalyst, there are those instances where active sites of the catalyst are covered with the coke so that the decomposition rate of acetaldehyde is lowered. The coke is catalyst poisoning, but can be removed by performing a catalyst recovering process to heat it at, for example, 200 to 400° C. in a heating furnace. However, the gas purification process should be interrupted to take out the photocatalyst from the reactor for performing such a catalyst recovering process, which takes time and labor. Further, in order to maintain the decomposition rate of acetaldehyde at a certain level, frequency of carrying out the catalyst recovering process may be increased to suppress adhesion of the coke on the catalyst surfaces.